A wheel-drive work vehicle includes an engine, a drive shaft that rotates by a drive force generated by the engine, and an axle apparatus coupled to the drive shaft (for example, refer to Patent Literature 1). The axle apparatus includes an axle housing, a differential that is housed inside the axle housing and coupled to a drive shaft, a transmission shaft to which a rotating force of the drive shaft is transmitted via the differential, and an axle shaft to which a rotating force of the transmission shaft is transmitted. Rotation of the axle shaft drives the wheels of the work vehicle. Machine parts such as a gear and a bearing that are housed inside the axle housing are immersed in oil for lubrication. Foreign substances such as a part of worn machine parts are mixed into the oil with the operation of the work vehicle. Therefore, the oil inside the axle housing is discharged by an oil circulating device, and passed through a filter for removing foreign substances. Then, the oil is returned to the inside of the axle housing again.